1. Field of the Invention
This invention relates to cellulose fiber reinforced cement composite materials using biocide treated cellulose fibers, including fiber treatment methods, formulations, methods of manufacture and final products with improved material properties relating to the same.
2. Description of the Related Art
Ordinary Portland cement is the basis for many products used in building and construction, primarily concrete and steel reinforced concrete. Cement has the enormous advantage that it is a hydraulically settable binder, and after setting it is little affected by water, compared to gypsum, wood, wood particle boards, fiberboard, and other common materials used in building products. The high pH of cement usually provides cement products good resistances to the damages by biological attacks.
Asbestos Fiber Cement Technology
About 120 years ago, Ludwig Hatschek made the first asbestos reinforced cement products, using a paper-making sieve cylinder machine on which a very dilute slurry of asbestos fibers (up to about 10% by weight of solids) and ordinary Portland cement (about 90% or more) was dewatered, in films of about 0.3 mm, which were then wound up to a desired thickness (typically 6 mm) on a roll, and the resultant cylindrical sheet was cut and flattened to form a flat laminated sheet, which was cut into rectangular pieces of the desired size. These products were then air-cured in the normal cement curing method for about 28 days. The original use was as an artificial roofing slate.
For over 100 years, this form of fiber cement found extensive use, for roofing products, pipe products, and walling products, both external siding (planks and panels), and wet-area lining boards. Asbestos cement composite was also used in many applications requiring high fire resistance due to the great thermal stability of asbestos. The great advantage of all these products was that: they were relatively lightweight; water affected them relatively little, and they had a good resistance to biological damages, since the high-density asbestos/cement composite is of low porosity and permeability. Asbestos fiber cement composites also have pretty good biological resistance. The disadvantage of these products was that the high-density matrix did not allow nailing, and methods of fixing involved pre-drilled holes.
Although the original Hatschek process (a modified sieve cylinder paper making machine) dominated the bulk of asbestos cement products made, other processes were also used to make specialty products, such as thick sheets (for example, greater than 10 mm which required about 30 films). These used the same mixture of asbestos fibers and cement. Sometimes some process aid additives are applied in the processes such as extrusion, injection molding, and filter press or flow-on machines.
Two developments occurred around the middle of the last century that are of high significance to modern replacements of asbestos based cement composites. The first was that some manufacturers realized that the curing cycle could be considerably reduced, and cost could be lowered, by autoclaving the products. This allowed the replacement of much of the cement with fine ground silica, which reacted at autoclave temperatures with the excess lime in the cement to produce calcium silica hydrates similar to the normal cement matrix. Since silica, even when ground, is much cheaper than cement, and since the autoclave curing time is much less than the air cured curing time, this became a common, but by no means universal manufacturing method. A typical formulation would be 5-10% asbestos fibers, 30-50% cement, and 40-60% silica.
The second development was to replace some of the asbestos reinforcing fibers by cellulose fibers from wood or other raw materials. This was not widely adopted except for siding products and wet-area lining sheets. The great advantage of this development was that cellulose fibers are hollow and soft, and the resultant products could be nailed rather than by fixing through pre-drilled holes. The siding and lining products are used on vertical walls, which is a far less demanding environment than roofing. However, cellulose reinforced cement products are more susceptible to water induced damages and biological attacks, compared to asbestos cement composite materials. A typical formulation would be 3-4% cellulose, 4-6% asbestos, and either about 90% cement for air-cured products, or 30-50% cement, and 40-60% silica for autoclaved products.
Asbestos fibers had several advantages. The sieve cylinder machines require fibers that form a network to catch the solid cement (or silica) particles, which are much too small to catch on the sieve itself. Asbestos, although it is an inorganic fiber, can be xe2x80x9crefinedxe2x80x9d into many small tendrils running off a main fiber. Asbestos fibers are strong, stiff, and bond very strongly with the cement matrix. They are stable at high temperatures. They are stable against alkali attack under autoclave conditions. Asbestos fibers are also biologically durable. Hence, asbestos reinforced fiber cement products are themselves strong, stiff (also brittle), and could be used in many hostile environments, except highly acidic environments where the cement itself is rapidly attacked chemically.
Alternative Fiber Cement Technologies
In the early 1980xe2x80x2s, the health hazards associated with mining, or being exposed to and inhaling, asbestos fibers started to become a major health concern. Manufacturers of asbestos cement products in the USA, some of Western Europe, and Australia/New Zealand in particular, sought to find a substitute for asbestos fibers for the reinforcement of building and construction products, made on their installed manufacturing base, primarily Hatschek machines. Over a period of twenty years, two viable alternative technologies have emerged, although neither of these has been successful in the fall range of asbestos applications.
In Western Europe, the most successful replacement for asbestos has been a combination of PVA fibers (about 2%) and cellulose fibers (about 5%) with primarily cement, about 80%. Sometimes the formulation contains 10-30% inert fillers such as silica or limestone. This product is air-cured, since PVA fibers are, in general, not autoclave stable. It is generally made on a Hatschek machine, followed by a pressing step using a hydraulic press. This compresses the cellulose fibers, and reduces the porosity of the matrix. Since PVA fibers can""t be refined while cellulose can be, in this Western European technology the cellulose fiber functions as a process aid to form the network on the sieve that catches the solid particles in the dewatering step. This product has reasonably good biological durability due to its high density and non-biological degradable PVA fiber. The major application is for roofing (slates and corrugates). It is usually (but not always) covered with thick organic coatings. The great disadvantage of these products is a very large increase in material and manufacturing process costs. While cellulose is currently a little more than asbestos of $500 a ton, PVA is about $4000 a ton. Thick organic coatings are also expensive, and the hydraulic pressing is a high cost manufacture step.
In Australia/New Zealand and the USA, the most successful replacement for asbestos has been unbleached cellulose fibers, with about 35% cement, and about 55% fine ground silica, such as described in Australian Patent No. 515151 and U.S. Pat. No. 6,030,447, the entirety of which is hereby incorporated by reference. This product is autoclave cured, as cellulose is fairly stable in autoclaving. It is generally made on a Hatschek machine, and it is not usually pressed. The products are generally for siding (panels and planks), and vertical or horizontal tile backer wet area linings, and as eaves and soffits in-fill panels. The great advantage of these products is that they are very workable, even compared to the asbestos based products, and they are low cost.
However, cellulose fiber cement materials can have performance drawbacks such as lower rot resistance and poorer long-term durability compared to asbestos cement composite materials. These drawbacks are due in part to the inherent properties of natural cellulose fibers. Cellulose fibers are comprised of primarily polysaccharides (cellulose and hemicellulose) and are highly hydrophilic and porous, which in combination make them an attractive source of nutrients for many microorganisms. As such, cellulose fibers are susceptible to bio-decay or rot attack when incorporated into fiber reinforced cement composite materials, which also happen to be highly porous. Particularly in high humidity environments, the pore spaces in the fiber reinforced cement material facilitate water transportation to the fibers and thus provide access to microorganisms such as fungi, bacteria, algae, and molds. Microorganisms can be carried by water through the pores of the cellulose fibers. The organisms can grow on the surface and/or inside the composite material by utilizing cellulose and hemicellulose as nutrients. The microorganisms will break down cellulose polymer chains, resulting in significant loss in the fiber strengths. The cleavages of cellulose fiber chains by the microorganisms eventually reduce the reinforcement efficiency of the fibers and adversely affect the long-term durability of fiber cement materials.
To summarize, the replacement of asbestos in Europe has been largely by air cured fiber cement products, using PVA fibers, and pressed after forming in the green state. The primary problem with this technology is increased material and manufacturing cost. The replacement of asbestos in USA and Australia/New Zealand has been largely by autoclaved fiber cement products, using cellulose fibers, and formed with lower density without pressing. However, the problems associated with this technology include higher porosity of the product and higher susceptibility to biological attacks when compared to asbestos fiber cement materials.
Accordingly, there is a need for a cost effective, fiber cement composite material that has improved rot resistance. There is also a need for an individualized reinforcing fiber that retains the advantages of cellulose and yet is more durable than regular cellulose fibers. To this end, there is a particular need for a more cost effective and durable fiber reinforced cementitious material that is resistant to microorganism attacks even in high humidity environments.
Applicant is aware of only one prior art reference that discloses applying a biocide agent to a cellulose fiber for application in calcium carbonate products (see U.S. Pat. No. 6,086,998). This patent is directed to making nonflammable cellulose fiber with addition of a small amount of xe2x80x9csurface-activexe2x80x9d biocide agents to the outer surfaces of the cellulose fibers. The ""998 patent is not specifically directed to the use of the fibers for fiber reinforced cement composite materials.
The above described needs are addressed by the preferred embodiments of the present invention in which partially or completely delignified and individualized cellulose fibers are pre-treated with selective inorganic or organic biocides, thereby producing an engineered cellulose fiber. When used in fiber cement composite materials, this engineered fiber retains the advantages of regular cellulose fibers of refining, autoclaving, and manufacture without pressing, but the resultant fiber cement material also can approach or even exceed the performance advantages of artificial fibers such as PVA, in terms of biological durability when used in fiber reinforced cement composite materials. The enhancement in the desirable biodurability is accomplished without any significant reduction in the important physical properties of the material, such as strength and toughness.
Thus, the preferred embodiments of the present invention disclose a new technology of making reinforced cementitious composite materials using biocide treated rot-resistant, individualized cellulose fibers. This new technology includes the following aspects: fiber treatment, formulations, methods of making the composite materials, and final materials and properties. The preferred embodiments of this invention solve the problem of poorer biodurability associated with cellulose fiber reinforced cementitious composite materials when compared with asbestos cement materials.
Thus, the use of these engineered rot-resistant fibers imparts to the composite material the enhanced biodurability properties, and therefore constitute an alternative technology that, when fully implemented, has the potential to maintain mechanical properties and the workability with the material in building and construction, while improve the durability of the products in the high humidity and rot-prone environments, regardless of the means of manufacture. They are particularly suitable to the Hatschek process that requires a refine-able fiber (to catch solid particles) and to the autoclave curing cycle that allows the replacement of cement with fine ground silica, although they may also be of use in the air cured products, in conjunction with PVA, to reduce the necessity of the expensive process pressing.
A composite building material made in accordance with one preferred embodiment of the present invention comprises a cementitious matrix and chemically treated and individualized cellulose fibers incorporated into the matrix to improve the biological durability of the final product. The inner and outer surfaces of the fiber cell walls are at least partially treated with chemicals (biocides) that inhibit microorganism growth. The chemicals may comprise inorganic compounds, organic compounds, or combinations thereof. The chemicals may include various kinds of fungicides, algaecides, and termite preservatives. Preferably, the chemicals comprise about 0.01% to 20% of the oven dry weight of the cellulose fibers.
Embodiments of the present invention will impart the fiber cement composite material with improved biodurability. Incorporation of the biocide treated fibers will increase the retention of the cellulose fiber when the fiber cement matrix is subjected to rot-prone high humidity environment. In one embodiment, the loss of fibers over 6 months of underground exposure was reduced from about 78% to about 32% when the biocide treated fibers are used. The high retention of fibers is indicative of better retention of reinforcement efficiency of the fibers in the fiber cement composite materials.
In another aspect of the present invention, a material formulation used to form a composite building material comprises a cementitious binder and cellulose fibers, wherein the cellulose fibers have been individualized and wherein at least a portion of the individualized fibers are pre-treated with at least one biocide such that the biocide inhibits microorganism growth in and on the fibers. A composite material formulation using the biocide treated fibers in accordance with one preferred embodiment comprises a cementitious binder, usually Portland cement; an aggregate, usually silica which may be fine ground if the autoclave process is used; individualized cellulose fibers wherein at least a portion of the individualized fibers are pre-treated with at least one biocide such that the biocide inhibits microorganism growth in and on the fibers; a density modifier; and additives. In one embodiment, the building material formulation preferably comprises about 10%-80% cementitious binder, more preferably about 15%-50%; about 20%-80% silica (aggregate), more preferably about 30%-70%; about 0.5%-20% biocide treated, rot-resistant, and individualized cellulose fibers, or a combination of rot-resistant individualized cellulose fibers, and/or regular cellulose fiber, and/or natural inorganic fibers, and/or synthetic fibers; about 0%-80% density modifiers; and about 0-10% additives.
In another aspect of the present invention, a method of manufacturing a fiber reinforced composite building material is provided. Individualized cellulose fibers are provided. At least a portion of the cellulose fibers is treated with a chemical, wherein the chemical inhibits microorganism growth in the treated cellulose fibers. The treated fibers are mixed with a cementitious binder to form a fiber cement mixture. The fiber cement mixture is formed into a fiber cement article of a pre-selected shape and size. The fiber cement article is cured so as to form the fiber reinforced composite building material.
Some of these steps can be omitted or rearranged, and other steps may be provided. The step of treating the fibers comprises treating the fibers with inorganic and organic biocides, or combinations thereof by means of techniques such as chemical reactions and/or physical deposition processes such as pressure or temperature impregnation and concentration diffusion. In this step, partially or completely delignified and individualized cellulose fibers are used for the fiber treatment. The effective biocides are attached to the fibers to provide enhanced biological resistances. The biocides that can be used for this purpose include a number of inorganic and organic chemicals and the combinations thereof.
Preferably, the step of mixing the treated fibers with ingredients to form a fiber cement mixture comprises mixing the treated fibers with a cementitious binder, aggregate, density modifiers, and additives. Preferably, the step of mixing the biocide treated fibers with ingredients to form a fiber cement mixture comprises mixing the biocide fibers with non-cellulose materials such as a cementitious binder, aggregate, density modifiers, and additives in accordance with the preferred formulations. In another embodiment, the biocide treated fibers can also be mixed with conventional untreated fibers and/or synthetic fibers, and/or natural inorganic fibers along with the other ingredients. The composite materials can be fabricated using any of the existing technologies, such as Hatcheck process, extrusion, and molding, etc.
Incorporation of the biocide treated fibers in the fiber cement matrix in accordance with the embodiments of the present invention improves rot resistance and durability of the final composite materials. The scope of the invention is not limited to particular types of cement, aggregates, density modifiers or additives, nor to their ratios in the formulations. These and other objects and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.